Return `id` assumption for simple polymorphic `Collection#model` factories.

Also, add tests to make sure it doesn't break in the future.
Make new tests more thorough
remove undelegateEvents call from remove
Merge branch 'master' of https://github.com/jashkenas/backbone into view-native-hooks
Merge branch 'view-native-hooks' of https://github.com/akre54/backbone into view-native-hooks

Conflicts:
	backbone.js